On Various Kinds of Dielectric Barrier Discharges

Contrib. Plasma Phys. 36 (1996) 6 , 697-709

On Various Kinds of Dielectric Barrier Discharges

S. MULLER, R.-J. ZAHN

Institut fiir Niedertemperatur-Plamaphysik e.V., Robert-Blwn-Str.8-10, D-17489 Greifswald

Summary

Dielectric barrier discharges in Xe and Ne/Xe, He/Xe mixtures are investigated with regard to the ranges of existence of various discharge types over a wide range of parameters (total gas pressure p , electrode distance d, frequency f and mixture ratio X). The discharges were explained in more detail because of the bistable behaviour and the transfered charges, partly of the peak current and the maximum of the visible radiation pulse. The time behaviour of the discharge was observed, too. In this paper a distinction of various discharge types could be achieved by the similarity parameters f/p and pd. The charge carriers remaining in the volume and long-living plasma species could be identified as essential mechanisms for the development of various types of discharges. The influence of space charges on the bistable behaviour could be shown indirectly.

1 Introduction

Dielectric barrier discharges (further called DBD) have been known under several names (silent discharge, barrier discharge, discharge between insulated electrodes, ac-discharge, normal pressure glow discharge, ozonizer discharge, display discharge, discharge in insulating cavities etc.). Detailed studies can be found in e.g. [l-391. The number of various names is mainly due to specific investigation methods or applications. The usual fields of technical application for the DBD are radiation sources [15--291 and plasmachemical reactors [18; 19, 30-371 in the range of normal pressures. DBD are used as radiation sources especially for UV-technologies. Incoherent excimer radiation is coupled directly out of the corresponding discharge arrangements in most cases and is used for photoche- mical processes. The short-wave UV- or VUV-radiation is transformed via phosphors into the visual spectral range in the case of colour displays (ac-plasma displays). With monochrome displays the visual spectral part of the discharge (mostly of Ne) is often used directly for representations. Well known, too, is the ozone generation in DBD [18, 19, 391, one of the most important examples of plasma chemical synthesis. In addition, the potential of these discharges for the destruction of pollutants or of toxic molecules has been demonstrated in many contexts' earlier 130-37). The common feature of all these discharges is that they are based on the ac discharge between insulated electrodes in the range of medium or normal pressure.

- optically diffuse and continuous for a total half wave of the sustaining voltage - optically not structured and pulse-shaped - glow like structure and pulse-shaped - contracted structure (filament) and pulse-shaped.

The following appearances of discharges between insulated electrodes are known:

698 Contrib. Plasma Phys. 36 (1996) 6

In addition, spreading effects on the insulation surfaces can influence the appearance of the discharge. Sliding discharges on the dielectric surfaces, mainly for the latter type, are described (Lichtenberg figures). Appearances can be observed occasionally which have to be assigned to the pulsed corona discharge (discharge systems with an inhomo- genous initial electrical field). In addition ranges of active discharges (e.g. contracted ones) can change spatially and temporally with non-active ones depending on the discharge history when using reactor or lamp arrangements (large-surface systems).

The above mentioned phenomena are naturally dependent on the plasmaphysical parameters and plasmachemical reactions. The properties of the plasma source are then determined by choosing parameters, especially the external ones. In connection with investigations on discharges, and used in monochrome plasma displays, we described earlier the first three above explained discharge types in Ne and Ne/N*, Ne/Ar mixtures, namely the pulse mode, continuous mode and quasiglow mode [9-111. We could show that a representation of the existence regions of these discharge types was possible using similarity parameters for a wide range of parameters.

The aim of this paper is to characterize the DBD on the basis of the dependence of some discharge phenomena on the external parameters in pure Xe and NefXe, He/Xe mixtures. These gases were chosen because Xe with its short-wave VUV-radiation represents an important gas for excimer radiation sources namely in lamps as well as for display purposes. Moreover, the ranges of existence of discharge types in these gases were examined. The application of the similarity parameters are dealt with for the representation of the existence regions of discharge modes.

2 Experimental Realization

2.1 Discha rge Vessel C o n f i g u r a t i o n s a n d G a s Hand l ing

As explained in the beginning, there can be complex combinations of various effects in large-surface discharge arrangements. Therefore we constructed two special discharge vessel configurations for the investigations, large-surface multi-element arrangements as well as single discharge arrangements, both in different special variations according to the aim of the measurements.

We used large-surface multi-element arrangements for the creation of application-like conditions. A planar type is shown in Fig. 1. The electrodes cover only a part of the dielectric. The gas space is situated between the dielectrics. We made the planar arrangement in Fig. 1 of Burgau quartz bulbs. The quartz plates (thickness 1.5 mm) were melted at the edge. The electrodes consisted of a wire screen attached by a silver solu- tion or of a bar electrode, fixed with a holding device. The latter is equivalent to a single discharge configuration but with a large-surface dielectric.

Several electrode arrangements were examined for the diagnostics in single discharges. One aim was to restrict large-surface interaction effects, another one was to

Fig. 1. Planar dielectric barrier dischare vessel.

S. MULLER, R.-J. ZAHN, Dielectric Barrier Discharges 699

7 , I - Z - U glass insuintion

Fig. 2, Geometry of the single discharge configuration.

get spatially and temporally stable discharges. A typical configuration for the generation of single discharges is shown in Fig. 2. Molybdenum was used as the material of the electrodes. These electrodes are flat on the front surfaces and conically shaped in the direction of the front surface. Thereby, field distortions shall be prevented at the edge of the discharge cell, which easily lead to spatial jitter effects. The Mo-electrodes were covered with Rasotherm glass and planely smoothed on the front surfaces, so that an insulation layer thickness of 0.4 mm was left there (in some cases 0.2 mm thick insulation layers were used). The mechanically smoothed surfaces were later flame polished. The other typical geometrical dimensions were : electrode diameter 2 111111, front surface diameter of the electrodes 1 mm and gas space thickness (electrode distance) 1.5 mm. Any variations in these parameters will be given with the respective results.

In addition to this construction we applied single glazed Mo-electrodes with a diameter of l mm and glazings with a diameter of 2 mm and a thickness of 0.2 mm on the front surfaces, too. The discharge vessels constructed in this way were graded in their discharge space length and were attached on a pumping and mixing device. This way, a quick variation of pd (gas pressure p , electrode distance d) was possible.

Concerning the vacuum technology the cleaning of the discharge vessels and the p r e cedure of filling and mixing were most important. The usual vacuum procedure for the discharge vessels was a heating process of about 7 hours at 410 "C. This step, important for the cleanness of the discharge vessels, was not possible for all configurations pre- viously described, because the windows, seals or flanges on the measurement devices did not allow any high temperatures. The influences of impurities in the remaining gas was lower due to the high pressure of the filling gas. The vessels were pumped down to 10-6mbar before filling. Spectroscopically pure gases of the purity classes of 6.0 to 4.8 respectively were used.

2.2 Power S u p p l y a n d C u r r e n t M e a s u r e m e n t

The following sinusoidal power supplies were used in the framework of these tasks: high frequency amplifier, transformer and final stage in resonance. The high frequency amplifier was a commercial device HPG2 of the EN1 company for the frequency range of 125-375 kHz. This one delivered powers of up to 200 W. Frequencies of up to 150 kHz could be used by inductive coupling for the measurements when not being operated too long. Another variation of the ac-power supply could be made by a final stage in resonance. The signal, taken from an HF generator in the frequency range of 15 kHz up to 1 MHz is amplified in two stages. The final stage has a series resonance circuit in the output. The inductance of the resonance circuit is grounded so that a non-symmetrical operation results. The operation at different frequencies is done by choosing new resonance circuits with a capacitive frequency fine tuning.

The temporal behaviour of the current has been measured. Its peak value and its time integrated value have been considered in more detail. The conclusion with regard to resulting wall voltages is complicated, because the capacity of the discharge cell is time- dependent in many cases due to spreading effects.


The current integration was done from current oscillogrammes by the menu pro- grammes of the used oscilloscopes. The voltage supplies delivered non-symmetrical sig- nals so that one electrode could be grounded via a resistor of 50 Q.

It was a problem for time-resolved measurements that the real discharge cell formed an LC circuit. We tried to work low-inductively if possible. But this was restricted in connection with the real construction of the discharge vessel. The capacitive displacement current was compensated in some cases by a bridge arrangement. There, a variable capacity of about the same size is switched parallel to the discharge cell and the discharge current is coupled out by a differential amplifier.

In many cases we could work without a bridge arrangement because the discharge duration is extremely short in relation to the half wave of sustaining voltage. Under such conditions the discharge current is clearly distinct from the capacitive displacement current. The voltage measurements were done with a capacitive voltage division of 1 : 1000. The voltage data shown in the results correspond to the exterior voltages, meas ured above the insulation. The static cell voltage Uc can be determined by the series connection of the three capacitors formed by the two insulation layers and the gas space. A possibly high bandwith of the measuring system was important. A 4-channel digital oscilloscope of Tektronix (TDS 644A) having a bandwith of 500 MHz and a Sam- pling rate of 2 Gs/s has been used.

3 Results and Discussion

3 . 1 B i s t a b l e R a n g e s a n d D i s c h a r g e T y p e s in P e n n i n g M i x t u r e s

Former investigations on DBD revealed a number of complex appearances of these discharges with partly different discharge types in a wide parameter range. Here measurements in Ne/Xe and He/Xe mixtures are presented related to this problem. Single discharge configurations were applied in order to prevent spreading or sliding discharge effects on the insulating surfaces to a large extent.

Fig. 3 shows the influence of some parameters on the berakdown and minimum sustaining voltage behaviour by the example of Ne/Xe mixtures as a function of the frequency. The .breakdown voltages Uz are marked by crosses and the minimum sustaining voltages UL by open circles. It is seen that the breakdown voltages (upper part of the curves) and the minimum sustaining voltages (lower part) are different at lower frequencies. This behaviour is called bistability. The discharge has a pulse-shaped character (pulse mode) in this parameter range. The bistable behaviour is mainly caused by the build-up of wall charges by carrier transport to the insulation layers of the electrodes. This wall voltage compensates the sustaining voltage and leads to pulse discharges, but adds to the voltage in the next half wave resulting in bistable behaviour. That means that the external voltage can be reduced to a certain value (the minimum sustaining voltage) without extinction of the discharges. This effect is used as an internal memory in ac plasma displays.

At higher frequencies the bistable behaviour is lost. In that case the voltages Uz and UL are nearly identical. There is a discharge with a lower current with regard to the above explained discharge type, with a nearly continuous character of the discharge activity for the complete half wave of sine sustaining voltage which has the features of a Townsend discharge (continuous mode). It has to be noted that there is a transition into a discharge type with a higher current at voltages above Uz. This form has a clearly axial structure and is therefore called quasiglow mode. This discharge type is bistable, too. The transition into this discharge type also depends on various other parameters

S. M ~ L L E R , R.-J. ZAHN, Dielectric Barrier Discharges 701

a-ao w u b-40 kPQ c-2 6.7 k Pa d - 1 3.3 k Pa

Fig. 3. Breakdown voltage UZ and minimum sustaining voltage UL vs frequency for Ne/Xe mixtures (d = 0.4 mm): ,

like mixing ratio and electrode distance. In large-surface multi-element arrangements the above explained frequency behaviour is observed only when all parallel-operated discharges have parameters of the same kind and the interaction between them is mini- mized, too.

Moreover, Fig.3 shows that the mentioned transition from the pulse mode to the continuous mode depends both on the pressure and on the mixing ratio. This can be seen on the frequency axis from the shift of the first measured point, where Uz and UL

l::I , ,] VISla.u.1

0 10 100 1000

fIkHz1

Fig. 4. Frequency dependence of the bistable range and visible light peak for He/Xe


are identical. This frequency is called here transition frequency ft between the discharge modes.

We can find a similar bistable behaviour in He/Xe mixtures. When the admixture part of Xe is higher, the above explained behaviour will not be clearly seen. Often the mentioned quasiglow mode ignites at first and then turns into the continuous mode with a voltage reduction. Such a situation is represented in Fig. 4. There the transition into the continuous mode is marked by squares. The bistable behaviour of the quasiglow mode, existing in higher voltages, is due to the wall voltage and to space charge influences as well. The clearly axial structure of the discharge gives evidence for that space charge disturbance. The maximum of the visible radiation pulse, measured by a photo multiplier, is shown among others in Fig.4 with a voltage kept constant within the bistable range. The intensity jump is clearly seen at higher frequencies, what corresponds to the higher current discharge.

In the following we focus on the transition frequencies into the continuous mode. There we use the similarity parameters f / p and pd. This way a connection between the transition frequency ft and the frequency f of the sustaining voltage, the gas pressure p and the electrode distance d is obtained. There, we take into account only voltage ampli- tudes of the sustaining voltage which are in the range of Uz and UL according to Figs. 3 and 4. Other parameters like the layer thickness d, of the dielectric or the diameter D of the electrodes are kept constant.

A distinction of the existence regions of the different discharge types can be obtained with the similarity parameters f / p and p d for the transition behaviour into the continuous mode explained above. Fig. 5 shows this similarity representation for the transition of the discharge types over a wide parameter range of the mixing ratio. The curves represent the respective transition into the continuous mode in the pressure normalised frequency depending on pd. A qualitative similar behaviour can be found in He/Xe. Fig. 6 shows an example for He + 2% Xe.

We can see from the figures that each mixture ratio has its own limiting curve with the corresponding pressure normalised transition frequency. In partial ranges of p d this

\q-x 0.03%

104 10 pd Pan]

Fig. 5. Existence regions of discharge types fbr Ne/Xe mixtures using similarity parameters ( x 80 kPa, V 75 @a, 0 40 kPa, 0 26.7 @a, 13.3 kPa).


f/p IHz/Pal

\+ 0 0 a *

lob

He+P%Xe + 10 kPa a 20 kPa o 40 kPa +% 50 kPa

I 4 I , , , , , I I I 1 I , I l l

1 10 100 pd [Pam!

Fig. 6. Existence regions of discharge types for He + 2% using similarity parameters.

is shifted to higher f/p values proportionally with the mixing ratio. On the other side the transition with higher pd-values with various mixture ratios is independent of them, whereas in lower pd-values an increase of f t / p is noted. The figures show that there is a bistable discharge type below the drawn limiting curves (pulse mode or quasiglow mode) and above them the continuous mode.

We have already referred to mechanisms for this transition behaviour in investigations e.g. in Ne/N2 f9]. The proportionality of the curves of the existence range to the mixing ratio is caused by the Penning effect, especially the lifetime of metastable atoms. The latter is determined by a number of different processes. By approximation we can as- sume under the present conditions that the destruction of metastable atoms by collisions with impurity gas atoms (admixed gas) is dominant in contrast to processes of loss by diffusion, two-body and three-body collisions of heavy particles and electron collisions. The following relation can be deduced:

tm - lifetimes of the metastable atoms tp - lifetime of the metastable atoms by the Penning effect X - mixing ratio N - gas density Zp - collision rate From this relation ft/p = const. can be deduced at a constant mixture ratio.

At the same time the Penning effect as a main process of loss of the metastable atoms is a very effective ionization mechanism which enables the sustaining of the discharge with relatively low voltages.

The break in the limiting curves is caused by the drift of the heavy particles because the carrier losses play an important role for the maintenance of discharge. Under the


present conditions the ions cannot leave the discharge volume. In this case the transition frequency will be independent on pd. Above a critical pd-value the ions reach the electrode walls. Below this critical value the transition frequency increases in order to com- pensate the ion losses.

In the range of the breakdown voltage a transition frequency normalized to pressure can be determined, where the ions still can leave the discharge volume. This is

f J P = (1/24 (bin,) uo/ ( P V with fi - ion transition frequency Uo - amplitude of the sustaining voltage b - mobility of the Xe-ions in the carrier gas ng - gas density at 1 torr and 0 "C This normalized ion transition frequency nearly corresponds to the break points in Figs. 5, 6 in the range of the breakdown voltage with drift conditions free of space charge. That means that below the line formed by the break points of f , / p the ions still can leave the discharge volume in the half wave but not above it.

For further details see similar investigations by us e.g. in Ne/N2 mixtures [9]. The following statements shall be pointed out as a summary for the further discussion

of results: 1) The drift of the ions in the electrical field influences the development of typical dis-

charge forms. 2) Long-living plasma species (e.g. metastable atoms) influences the developing dis-

charge forms too, which causes a dependence on the mixing ration. In this case the Penning effect plays an important role with the ionization processes.

The above mentioned statements have to be modified in the case of the pure noble gases or other mixtures (no Penning mixtures) because there are other dominating ionization and destruction processes. Furthermore it has to be noted that the simple drift model is valid only in the range of the transition into the continuous mode with the respective voltages. Analyses in more detail of the field in the discharge cell are necessary for higher voltages or in the range of the bistable discharge. The represented interpretation has to be seen as an approximation, where the good distinction of the existence ranges with the similarity parameters are surprising. .

3.2 M e a s u r e m e n t of S o m e E l e c t r i c a l P a r a m e t e r s i n Xe

The discussed bistable behaviour also occurs in pure Xe. Fig. 7 represents the bistable range of an Xe-discharge for a discharge system of the construction type according to Fig. 1 with a fixed single electrode. This dependence can in principle also be compared with that of a planar multi-element arrangement or that of a single discharge in Fig. 2. In addition, a filament.discharge type occurs here under the present parameters in contrast to the explained structures in mixtures which corresponds to a streamer discharge (this type is not restricted to pure Xe). We will discuss this structure further in another paper.

Fig. 7 shows a typical course (the measured points for later investigations are also drawn) : the minimum sustaining voltage decreases with a nearly constant breakdown voltage and with an increase of the frequency in this range, so that the bistable range of the discharge will become broader.

With increasing bistable range there is a noticeable frequency and pressure dependence (Figs. 8, 9), which should be caused by remaining charges or a remaining concentra-

3000

2500-

2000

1500

1000

500

705

X :: :: " X h

-

- a

-

-

10 20 30 40 50 60 70 80 90 100 110 120 f[kHzl

Fig. 7. Frequency dependence of the bistable range for Xe.

tion of excited species as well as higher concentrations of species due to increased charge carrier densities by space charge perturbation and/or by the pressure increase and high current densities, which contribute dominantly to the sustaining of the discharge mainly by two-body reactions and step ionization.

The determination of the voltages is difficult due to a dependence on the operation time (heating) as well as a long-term ageing which cause an increase of the voltages. The question of gas purity plays an important role, too. Both Fig.8 and 9 show the dependence on the similarity parameter pd. The other conditions for the applicability of this similarity parameter have not been examined here further.

2000 -

1000 -

0.1 1 10 100

pd [Pam]

Fig. 8. Dependence of the breakdown- and minimum sustaining voltage on pd for Xe at 20 lcHz (two measurements).

706

12

10

8 -

6 -

4 -

2 -

Contrib. Plasma Phys. 36 (1996) 6

-

U,/U,[kVpJ 5

4

3

2

1

0 0.1 1 10 100

pd [Pam]

Fig. 9. Dependence of the breakdown- and minimum sustaining voltage on pd for Xe at 100 kHz (two measurements).

The transfered charge as function of the frequency is shown in Fig. 10. The measuring points for the current integration are marked in Fig. 10. The values represent the aver- age of three successive current pulses. The figure shows that the amount of charge transfered per half wave decreases with increasing frequency. This is a clear hint on the fact, that the increase of the bistable range cannot be deduced from a wall voltage increase (or effects by wall voltages) with increasing frequency. A contrary tendency can be deduced from the measurements, whereas spreading effects have not been clarified. so that the above discussed interpretation is supported.

0 50 100 150 200 fIkHzl

Fig. 10. Dependence of the transfered charges on the frequency for Xe.


/

0 1,27 1,37 1,47 1,57 1,67 1,77 1.87 1,97 2,07 2,17 2,27 2,37

UlkVef,l

Fig. 11. Dependence of the transfered charges and peak current on the sustaining voltage for 50 kHz.

Fig. 11 and 12 show the influence of the sustaining voltage on the transfered amount of charge and the peak current value. The results are represented for frequencies of 50 kHz and 100 kHz, respectively. A proportional relation between peak current and transfered amount of charge can be seen in the lower voltage range. The peak current value is saturated with increasing voltage but the transfered amount of charge still increases. This is caused by a broadening of the current pulse as the number of discharges increases and temporally spreads. The bistable range is caused by various effects: the

QJn As1 121

UIkV,f,l Fig. 12. Dependence of the transfered charges and peak current on the sustaining voltage for 100 kHz.


wall voltage, space charge distortions, remaining charges in the volume and the densities of various species during the discharge. The wall voltage itself depends on the cell voltage during the discharge, the voltage pulse width (half wave duration), the polarity of the cell voltage related to the wall charge distribution and the kind of the wall charge distribution before the discharge.

4 Remarks and Outlook

In addition to the discussed results on the bistable behaviour and the transition to the continuous mode we have to consider another general aspect. The discharge develop ment in a sustaining voltage sequence will gradually intensify until a stable point is reached. This growing discharge sequence is also represented in the so-called charge transfer curve which describes the dependence of the wall voltage on the cell voltage [6, 71. There are two stable states in this curve. In the lower part only a small wall voltage is transfered and in the upper part there is a maximum wall voltage transfer. If the stabilization of the discharge is successful in the lower part of the curve, we have a low- current discharge like the continuous mode. The discussed long-living plasma species as well as ions remaining in the volume can be regarded as such stabilizing elements. In another case a sequence of cell firings will be generated which will result in a steady- state operation at a higher value where the pulse character dominates. That means that the wall charge transfer or space charge distortions are only one aspect of the bistable behaviour. .

The modelling of the discharge as well as time resolved measurements will be subject to a later publication for the comprehension in more detail.

Acknowledgemen t

The authors would like to thank the German Ministry of Research and Technology (BMFT) for partly financing this investigation.

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On Various Kinds of Dielectric Barrier Discharges